Quantified Risk Assessment Techniques - Part 2

Event Tree Analysis - ETA

Health & Safety Briefing No. 26b

February 2015

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Introduction

Quantified Risk Assessment Techniques (Part 1) discussed Failure Modes and Effects Analysis (FMEA).

This is the second briefing note to describe a specific risk assessment technique, Event Tree Analysis - ETA. It must be emphasised that this brief treatment is intended to be illustrative rather than definitive.

Event Tree Analysis (ETA)

Event tree analysis is based on binary logic, in which an event either has or has not happened or a component has or has not failed. It is valuable in analysing the consequences arising from a failure or undesired event.

An event tree begins with an initiating event, for example, a component failure, increase in temperature/pressure or a release of a hazardous substance. The consequences of the event are followed through a series of possible paths. Each path is assigned a probability of occurrence and the probability of the various possible outcomes can be calculated.

In the following example fire protection is provided by a sprinkler system. A detector will either detect the rise in temperature or it will not. If the detector succeeds the control box will either work correctly or it will not - and so on. There is only one branch in the tree that indicates that all the subsystems have succeeded:

Figure 1: Fire Protection System

Figure 2: Simplified Event Tree

The analysis can be quantified as illustrated in the following example related to a building protected by a sprinkler system:

Smoke Detector

Fire

Exit

AlarmSprinkler System

Fire Detected? Fire Alarm Works?

Sprinkler Works?

Limited Damage

Extensive Damage People Escape

Limited Damage Wet People

Possible Fatalities Extensive Damage

Initiating Event

Fire Starts

Y

N

Y

N

N

Y

Y

N

Resultant Event

Figure 3: Quantification of Event Tree for Building Protected by Sprinkler System

Application

ETA has proved to be a useful tool for major accident hazard assessment and was used by the UKs Health and Safety Executive, for example, for the assessment of risks to the public from serious accidents at installations in the Canvey Island area of Essex. In major accidents, ETA is used for the evaluation of possible consequences following a release of toxic/flammable vapour cloud from a process and to analyse the effects on plant, personnel, general public and the environment.

References and Further Reading

Risk Assessment Methodologies - H Raafat, University of Portsmouth, ISBN 1 069959434 DIN 25419 Event Tree analysis; graphical symbols and evaluation (in German) BSR/AIAA S-102.2.17-200x Performance-based Reliability and Maintainability Programs - Event Tree Analysis

These Briefings contain a summary of recent Health & Safety issues, provided for general information purposes only, and should not be relied upon as legal advice. The IET has tried to make the Briefings accurate and informative, but they have not been prepared by a lawyer and may not constitute an up-to-date summary of the law. The IET accepts no liability for your use of these Briefings. Further details and information on broader Health & Safety issues can be obtained from the Governments Health and Safety Executive. Legal advice should be obtained on any specific issues.

The IET is unable to provide further information on this topic. Please contact the HSE http://www.hse.gov.uk/

Y

Y

Y

N

N

NP = 0.1

P = 0.9

P = 0.7

P = 0.3

P = 0.5

P = 0.5

Fire Starts

Frequency = 1/yr

Initiating Event

Fire Spreads Quickly?

Sprinkler Fails To Work?

People Cannot Escape?

Resultant Event

Scenario

Multiple Fatalities

Loss / Damage

Fire Controlled

Fire Contained

1

2

3

4

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